It depends somewhat on your desired market for your themeless, but keep in mind just how competitive the themeless market is and how selective editors can be. In the last few years, I have often said to myself, "Give them NO reason to say "no' to this."

--Listen to your gut if you have any misgivings at all about a clunky partial or a semi-obscure abbreviation - I feel safe in saying that most themeless specialists have had something declined on the basis of one or two SHORT entries an editor had no interest in using/justifying, even if the longer entries/stacks they facilitated seemed to throw off triumphant sparks. Remember the creative-writing adage of "kill your darlings." Many times people post queries on here looking for reassurance -- or a veto -- about a given entry; just my opinion, but in most cases, you should already be working on alternate fill if you have any qualms at all.

--Check to make sure the percentage of proper names in the puzzle is not too high, and look vigilantly for any crossings that could be considered particularly difficult - where the entries going in either direction might be considered niche knowledge. You could also do a GENERAL review of your draft for specialized vocabulary. Some themeless solvers don't mind being taught a new word or two - I usually don't -- but others claim that it poisons their whole experience of the puzzle. The debate over what constitutes general knowledge will never be settled, so I'm not saying you need to jettison difficult vocabulary, just ponder it. I don't want to put words in Will Shortz's mouth but I'm nearly certain he has gone on record as feeling that it's problematic to restrict puzzle fill to only the most basic corpus of vocabulary and to rely solely on cluing/wordplay to impart difficulty.

--Presenting an absolutely clean themeless sometimes feels like watering it down and losing some sparkle in a certain area, and you're left fearing the dreaded "didn't excite me enough," rejection letter. But sometimes you can pack out puzzles with the fresh and the au courant and Scrabbly and get a verdict of "this feels like you're trying too hard." The next month you can get an excited "yes" on something you weren't as enamored of. Tinker, tinker, tinker - don't be content with what comes along soonest or easiest, as I say all the time to researching students of the Google generation. You can achieve the right balance between smoothness and panache. If you start off with two vibrant seed entries, each in an opposite corner, and you can't make things come together, simply shift the stack with an abandoned seed into a new puzzle starter without mourning The Masterpiece That Got Away. Don't get discouraged. Themeless appeal is very subjective. If you develop a knack for it - and if you learn to find satisfaction in the sort of claustrophobic revisions that various editors need as you shop the themeless around - I think you'll be happy with your track record.

--Make sure your grid design is not too "islandized." Make it very interlocking, an isthmus of one corner always penetrating the middle or an adjoining section, to decrease the solver's chances of grinding to a halt. Yes, it seems harder to complete, but I can think of many times where actually ADDING white squares to the grid unlocked a difficult section. Minimizing the number of three-letter words might also be a goal you could take on. I used to do quite a few themelesses with stacks of 11s, and I've virtually stopped because of the boredomof the three-letter words opposite.